Implantable electronic devices for neural recording and/or stimulation may include either insulated conductive microwires with exposed tips or micromachined neural probes with an array of microelectrodes. These devices are considered as an important neurotechnology for system neuroscience and neuroprosthetics. However, reactive tissue response to these devices deteriorates the device performance over time by compromising the recording stability, signal quality, and operating lifetime (their chronic or long-term performance). Thus, it is desirable to have probes designs which can suppress the reactive tissue response and/or reduce chronic tissue response.
The basic function of a neural probe is to introduce an array of microelectrodes into a neural tissue along with their electrical wiring. As the probe is physically inserted into the tissue, it is desirable that mechanical strength of the probe is greater than the force required to insert the probe into the tissue to prevent buckling or fracture of the probe. Moreover, during insertion, the probe may injure or destroy the tissue in a local area producing a “kill zone”. It is preferred that the kill zone is minimal and the nerve cells near the microelectrodes are preserved for an effective chronic recording and stimulation. To limit the kill zone solely to the area intersected by the probe, the surrounding tissues should be subjected to negligible stretching or compression during the probe penetration. This can be possible if the probe cuts its way through injuring only the intersected structures and preventing propagation of the damage into the surrounding tissues. This may then minimize the kill zone. Otherwise, stretching or compression may tear the neural tissue which leads to neural cell death and/or rupturing of blood capillaries. Bleeding due to rupturing of blood capillaries can cause extensive neuronal displacement or destruction. Thus, it is desirable to have a probe designed such that the probe can be inserted into the tissue without distorting or tearing the walls.
Conventional neural probes exhibit either a conical shape with round cross-sectional profile (as in “Utah Probe” or glass micropipettes) or a blade shape with rectangular or semi-circular cross-sectional geometry (as in “Michigan Probe”) [1], [2]. The conical shape may seem ideal for probe geometry since the sharp tip of the probe can create a tiny hole through which the probe can penetrate deeper into the tissue by gently pushing the adjacent tissue aside. This mechanism may work if the neural tissue were made of a homogenous elastically deformable material. However, the neural tissue is fibrous in nature being packed with myelinated axons, microtubules, and neurofilaments criss-crossing one another forming a “fishnet” like woven structure. A conical probe penetrating into the tissue would shear the tissue with the tissue elements eventually stretching and forming a fibrous band around the probe. With further penetration of the probe, the band becomes larger and tighter. Thus, additional force is required to cause the probe to penetrate further into the tissue. The additional force on the penetrating probe may spread to adjacent tissues as the band compacts. As such, the tissues adjacent to the band may be pulled in tension. When the tissue is pulled in tension beyond its elastic limits, membranes of neurons may rupture. Small blood vessels may also be torn and subsequent microhemorrhages may destroy or displace the neural tissue on a large scale. Thus, a conically shaped probe is probably unsuitable for atraumatic implant.
Blade type probes, depending on their microfabrication approach, may exhibit either a characteristic structure with thickness and width converging into a point-like sharp tip or a profile having a uniform thickness terminating at a sharp edge at the tip. In either geometry, the thickness of the blade type probe is considerably reduced as compared to its width. Thus, the blade type probe may ease the band of tight tissue that can tear as compared to the conically shaped probe. However, it is desirable to further reduce penetration trauma to the tissue and/or to prevent tearing of the tissue.